The present invention relates to a captive thread-forming screw assembly, and more specifically, to a captive thread-forming screw of the type employed to provide an electrical terminal on a bus bar or the like.
It is relatively common practice to employ self-tapping terminal screw in the manufacture of electrical devices so that wires or their metal terminals may be attached to a bus bar or workpiece with a threaded fastener without the necessity of machining internal mating threads in the workpiece to receive each particular fastener. During the manufacturing or assembly process, it is often necessary to retract the fastener with a power driver and to avoid backing the fastener completely out of the workpiece, which would result in an increase in the overall assembly time, captive screws are commonly employed. The captive screws used as terminals are generally of the type shown in FIG. 1 of the drawings and while widely used, this design does possess certain inherent disadvantages, which are overcome to a large extent by the present connector.
More specifically, with the prior art designs, the work entering end of the fastener diverges in a direction away from the driving head and is of a diameter greater than the root diameter but less than the crest diameter of the thread thereon. When the fastener is inserted into a workpiece having an apertural diameter less than the crest diameter of the threads and rotated, internal mating threads are cold formed on the periphery of the aperture with a crest diameter determined as a function of the root diameter of the male thread on the fastener. This results in the reduction of the effective diameter of the aperture sufficient to cause the tapered work entering end to impinge against the internal threads upon retraction, and render the fastener captive in the aperture. However, as these fasteners are retracted with power drivers during assembly or subsequent service, there is a distinct change that the assembler will over rotate the fastener. That is, back the fastener out of the aperture further than is required to attach a lead wire or terminal. When this occurs, the thread turns on the diverging end of the fastener will continue to maintain purchase or engagement with the workpiece thread. As such the enlarged end of the fastener is drawn back through the aperture causing the internal threads previously formed thereon to be destroyed or wiped out. If the over rotation is excessive, which can easily occur with a power driver, the fastener will be backed completely out of the workpiece. The present invention overcomes this problem and achieves other advantages not found in prior art fasteners by providing a design which will not result in damage to the apertural thread when backed out, even if over rotated. More specifically, the present invention provides a design having a tapered lead-in portion disposed intermediate to the thread forming portion and the enlarged work entering or abutment portion. The lead-in portion contains underfilled threads and converges in a direction away from the driving head. As is shown in FIG. 3 and FIG. 5, the underfilled lead-in threads run out on the converging section of the shank as said lead-in threads approach the enlarged abutment projection. This design allows the fastener to be rotated freely while fully retracted, yet captive without destroying the apertural threads in the workpiece. In addition, as this lead-in portion tapers and converges toward the work entering end, the initial engagement of the fastener is facilitated and the driving torque required to cold form the threads in the workpiece is substantially reduced.